Coal is a major energy source in China, and its reserve is about one trillion tons, counting for more than 95% of the total fossil fuels in China. On one hand, more than 84% of the coal in China is directly burned, resulting in the low heat efficiency and much pollution. On the other hand, there is an increasing demand for the natural gas in China. It is estimated that the demand for natural gas will be 200 billions cubic meters in the year 2020, but the natural gas production at that time will only be 140-160 billions. Moreover, the natural gas made from coal can be transported in the pipes on large scale, resulting in the energy cost reduction, environmental protection, safety and low transportation cost. Thus, making proper use of coal resource and developing advanced technology for converting coal into natural gas cleanly and effectively are of great importance.
Converting coal into combustible gas such as H2, CH4, etc. by use of supercritical water is an emerging technology. In this field, there have been many researches in the world, but till now none of them reached the pilot plant stage. The General Atomic Company in USA uses supercritical water to oxidize the aqueous coal slurry (40 wt %) to produce H2, but the results indicated that high concentration of aqueous coal slurry will result in the blockage and coking in the experiments. CCUJ Company in Japan uses supercritical water to oxidize the mixture of coal and catalyst such as CaO to produce H2, but this technology is not suitable for industrial production because the amount of catalyst used is too large. Xi'an Jiaotong University studied the co-gasification of coal and biomass. In patent CN1654313A, Guo Iiejin, et al., used the coal and biomass model and co-gasified many kind of biomass in the supercritical water, but the concentration of aqueous coal slurry is low (<2 wt %), so the energy cost in the conversion process is increased. Shan'xi Coal Chemistry Institute has done much work on the production of H2 from low-grade coal by the supercritical water oxidation (SCWO) method. In patent CN1544580A, Bi Jicheng et al., disclosed the conversion method of low-grade coal in supercritical water, but from the results of related experiments, the conversion of coal is lower than 50%, this is not suitable for industrial production. Furthermore, there is no report on producing methane from the coal in the supercritical water. To sum up, there are still some technical problems to industrially carry out the coal conversion in the supercritical water. The main problems are that the particle size of catalyst is large and the specific surface area of catalyst is small, and the catalyst particles can not uniformly disperse on the coal particles, which limit the contact of catalyst with coal, leading to the low catalyst activity. In traditional method, one generally increases the adding amount of catalyst to improve catalysis performance because of the low catalyst activity. The adding amount of catalyst is generally from 20 wt % to 40 wt %. This adding amount is so big that the flux of reactants has to be decreased, and the effective recovering and recycling of catalyst is a problem which is difficult to solve, and the producing cost is increased.